1. Field of the Invention
The invention concerns a process for the manufacture of polyurethane adhesives which are soluble in polar, inert organic solvents, and the molecular weight of which is accurately adjustable and reproducible.
2. Description of the Prior Art
The use of long-chain, primarily linear polyurethanes, possibly in combination with certain cross-linking agents, as adhesives is a familiar fact. The polyurethanes are generally supplied in granulate form and are used as adhesives after being dissolved in certain solvents, such as methyl-ethyl-ketone or ethyl acetate. For reasons of adhesion technology the solutions with a given solids content must have as accurately reproducible a solvent viscosity (also: solution viscosity) as possible, i.e., the polyurethanes must have as accurately reproducible a molecular weight as possible.
The manufacture of polyurethanes in solution by conversion of organic polyisocyanates with organic compounds, which contain reactive hydrogen atoms capable of reacting with the isocyanate groups, has been part of the state of the art for a long time. Such polyurethanes are used for a multitude of applications. However, it is very difficult to produce reproducible products with uniform physical properties with sufficient accuracy, particularly on an industrial scale, when the basic compounds for the polyurethane synthesis are reacted in approximately equivalent quantities, i.e., with an NCO/OH ratio of between approximately 0.95:1 and 1.10:1. The reason for the insufficient reproducibility of the properties of polyurethanes lies in a number of possible secondary reactions which depend upon a number of difficult to control parameters.
The most important of these parameters is the NCO/OH ratio itself. Under operating conditions this parameter can be maintained constant only within a range of .+-.1%. In order to obtain accurately defined and reproducible molecular weights in accordance with prior known techniques, a weighing accuracy for polyol and polyisocyanate in the range of 0.1% is essential. Even a slight variation in the NCO/OH ratio results in great changes in the rheologic properties, such as molecular weight, melting index, solution viscosity, content of highly cross-linked particles and therefore, finally, the quality of the adhesive.
The variations in the NCO/OH ratio can be caused by weighing or metering errors and also by insufficient purity of the reactive materials and the solvents, e.g., by water, or by slight variations in the molecular weight of the polyols.
In order to manufacture products of uniform and reproducible quality, it is therefore necessary to control the polyaddition process in such a manner that--upon reaching the desired degree of polymerization and/or the desired viscosity of the polyurethane solution--the polyaddition can be completed without interfering secondary reactions.
It is a known fact that the polyaddition process can be terminated by adding monofunctional H-acid compounds, such as primary and secondary alcohols or amines.
According to a process described in the German Patent Publication, DT-OS No. 1 595 784, a primary alcohol is added to a polyurethane solution in dimethylformamide in order to terminate the polyaddition reaction. An excess of alcohol must be used in order to obtain a quick breaking of the chain with alcohols. In the case of many polyurethane solutions, particularly in the case of polyester-urethane solutions this results in the decomposition of chains.
According to DT-OS No. 1 906 555 a quick chain breaking reaction by addition of a monoamine is possible, but it is very difficult to add exactly the required amount of monoamine. An excess of monoamine causes a decomposition of the chain, whereas too little monoamine causes continued chain expansion of the polyurethane.
In DT-OS No. 2 323 393 it is suggested that secondary alcohols be added to the reaction mixture in order to selectively suppress the formation of allophanate. This results in a reduced dispersion range of the viscosity of PUR solutions in case of variations of the NCO/OH ratio up to approximately 4%, and these variations are still too large for many areas of application.
In U.S. Pat. No. 3,310,533 it is suggested that the polymerization reaction be terminated by pouring out and rapidly cooling the reaction mixture. This process also is not suited for the production of polyurethanes with defined molecular weights, since the polyaddition products still contain reactive NCO-groups which react during storage, thereby enlarging the existing molecules.
According to DT-AS No. 1 694 127 monofunctional compounds in subordinate quantities (relative to the NCO content) can also be used in order to reduce the molecular weight and/or in order to still obtain soluble polyurethanes in spite of a certain branching of molecules. These monofunctional compounds include, e.g., butylamine, butyl semicarbazide or N,N'-dimethylhydrazine. This procedure necessarily results in great variations in the end viscosities of the PUR solutions. If the molecular weight of the polyurethane and/or the polyurea-urethane is too low, DT-AS 1 157 386 suggests adjusting the desired solution viscosity by means of carefully adding additional--preferably less reactive aliphatic di- or triisocyanates. After reaching the desired viscosity the product can be stabilized by converting the end groups of the polyurethane with monoisocyanates such as butylisocyanate, anhydrides, or other acylating substances such as acid chlorides. In addition to the laborious mode of operation, this process has the drawback of having the viscosity adjustment take place very slowly and the termination reaction with the end groups not take place instantaneously, therefore not resulting in an accurately controlled viscosity.